Band receiving system



Oct. 21, 1941. F. K. VREELAND ,25

BAND RECEIVING SYSTEM Original Filed Aug. 1, 1927 2 Sheets-Sheet; 1

IN V EN TOR.

0a. 21, 1941. K VR ELAND 2,259,595

BAND RECEIVING SYSTEM Original Filed Aug. 1, 1927 2 Sheets-Sheet a y II/k INVENTOR.

A TTORNEYS.

Patented Oct. 21, 1941 UNITED STATES PATENT OFFICE BAND RECEIVING SYSTEM Frederick K. Vreeland, Montclair, N. J., assignor to Vreeland Corporation, New York, N. Y., a corporation of New Jersey 2 Claims.

This application is a division of application Serial No. 377,409 filed July 11, 1929, now Patent No. 1,850,973, issued March 22, 1932, which is itself a division of application Serial No. 209,650, filed August 1, 1927, on which Letters Patent No. 1,725,433 issued August 20, 1929.

The invention herein described relates to a system of receiving alternating currents including a band of frequencies, particularly such a band of frequencies as comprise the transmission band of a modulated signal wave.

The general purpose of the invention is to receive the component frequencies of such a band with such uniformity as to avoid material distortion of the modulated wave, and to exclude frequencies outside of the band which the system is designed to receive. Another purpose of the invention is to provide means for shifting the position of the band in the frequency scale at will, by a simple adjustment, so that the system may be readily adapted to receive modulated waves of. any desired carrier frequency, including the side bands of such modulated waves. A particular object of the present invention is to secure the band characteristic in a single unit of a receiving or amplifying system, such unit giving substantially uniform reception for all frequencies within the band for which it is designed, with a sharp cut-off for frequencies outside the desired band, thereby securing in a single unit of the system a high degree of selectivity without distortion of the modulated signal wave. In one embodiment of the invention the band selector unit is combined with an antenna or other collector, and a compensating reactance is employed to compensate the indeterminate reactance introduced by the collector and preserve the necessary symmetry of the system. Other features of the invention relate to the combination of a plurality of such units, each having a band characteristic, in a receiving and amplifying system, giving a high degree of amplification over a band of frequencies with a high selectivity of power of excluding frequencies outside the desired band. Other desirable features of the invention are explained at length. The original application on which were issued Letters Patent No. 1,725,433 has been restricted to the band selector as a unit and specifically to that form of band selector unit in which the common reactance bridges the common terminals of the two reactive couples while the claims of the divisional application 377,409 (Patent 1,850,973) are directed to the combination of such a band selector with other elements of a receiving system. The claims of the present application are directed particularly to that form of .band selector unit in which the common reactance is a mutual inductance. Such mutual inductance, operating between the couples, may reside either in a separate mutual inductor or in a mutual inductive coupling between the Whole or any part of the inductances of the two reactive couples.

When selectivity, or the power of separating a signal wave of one carrier frequency from undesired waves of different carrier frequencies, is accomplished by the usual method employing a tuned circuit or circuits, the frequency characteristic of the receiver is essentially peaked, since there is only one frequency at which the capacity and inductance reactances of the circuits are balanced. At any other frequency there will be an unbalanced reactance in the system which cuts down the response to such frequency. In receiving a modulated wave, comprising a band of frequencies, such a system will receive one frequency of the band effectively, and the other frequencies of the band less effectively or not at all, with resulting signal distortion.

In the case where a plurality of synchronously tuned circuits are employed in cascade, in the usual way, selectivity is increased since the amplification at peak, frequency is increased in geometric ratio and the amplification at any other frequency is increased in a much smaller ratio, but this selectivity is necessarily secured at the expense of tone quality, since the side bands are relatively reduced according to the same law. It has been proposed to improve the reception of side bands by introducing damping into the synchronously tuned circuits, but this only results in partial mitigation of the distortion and this mitigation is gained at the expense of selectivity.

In my Patents Nos. 1,666,518, April 17, 1928, 1,682,874, September 4, 1928, and 1,730,987, October 8, 1929, I have described means whereby substantially uniform reception is obtained at all frequencies included in the band of a modulated wave, the means specifically claimed in these patents being the use of successive stages of amplification having different frequency characteristics, and in combination producing a band characteristic. 1

By means of the invention common to the Patent 1,725,433, the application 377,409 and the present application I am able to secure a similar uniform band characteristic in a single selector unit comprising a system of reactances so related quencies outside this band. When" thesystem is suitably constructed, as hereinafter described, the cut-off at the limits ofthebandis"- exceed ingly sharp. By the use of such a selector I am able to secure distortionless reception of the entire band of frequencies included in a modulated wave, and effectively eliminate the-tre quencies of interfering waves; Because: of the sharp cut-off this uniform Comparing the frequency characteristic of my selector unit with that of a pair of selective cir'-' cuits tuned ic -resonance the usual way itis foundthat the broadening ofthe band over the effective frequency range is accomplished with''- out ari'y increase of the width of the cur e at its base, which determines the selectivity of the systefri-.-

number of my band selector may be employed in cascade. In one arrangement that is especially effective they may be used for exampieas coupling units iha] multi=stage amen fier', thus securing increased signal strengthand increased selectivity Without impairing the uniformity of the band receptionand hence with out the increased distortion-by trimming the side bands which necessarily occurs when geometric tuning is employed.

The construction ofmy band selector is so simple, and the means for compensatirig irideterminate reactanc'e's so effective, that com lete symmetry or sirhi'larity may be readily secured in the several com onent circuit elements; so that'commohcontrol means may be eifectively applied to the frequency adjustment.

These features areil-lu'stratedand the apparatus employed is fully explained in the accompanym drawings and in the followiiig description. In the drawings: v

Figure 1 represents schematically one of my band selector units; in generalized form: Figure la represents schematically one of the specific forms of sum band selector unit.

Figure 2 is a vector diagram-showing the emtion of the currentsthe various parts of the system of- Figure 1. Figure 3 is a typical curve representing the frequency characteristic of one of my band s"elector u'riits.- It shows also for comparison a frequency characteristic of an ordinary tuned circuit. v I

Figure 4 shows aradio receivings'y'steri'i embodying one of my band selector units associated with an antenna or collectingcircuit on the one hand and an aperiodic amplifying and detect ing system on-the other;

I Figure 5 shows a band selector unit employed as a preliminary selector or pre selec'tor with a band amplifier;

Figure 6 shows a radio receiving system embodying a plurality of my band selector un ts, one beingassociatedwith a collectoras Figure 4 and the others beihgemployed as u plihg-units a mum-stage radio frequency am-- plifier;

5 Figure; 1 shows one or my h'ar'id selector" base reoeptrorr is accomplished without any loss of selectiyi-ty.

" lation to the reactances of the two reactive couples. It serves as a band forming reactance tending to balance the unbalanced portions of the two reactiv'e units and .renders the system responsive with substantial equality to all frequencies' within a band whose width depends upon the relative values of the band forming reactaric'e and the other reactances. For frequencies outside of this band, whether higher or lower than the frequencies included within the band, the unbalanced portions of the reactances of the two reactive couples become greater or loss than the efiecti-ve reactance of- Xs; whichis hence unable tobalance them so that the sys= tem as a whole has an over-all reactor-loo which prevents its transmission of currents ofoon frequencies outside the band. In the generalized form of the invefitioh the reactance X's-in'a'ybe uhturied and either an inductance, a capacitance; or amutualinductance- Figure 1" showing the re: actance in generalizedsymbolic form,

In the Pateht' 1 ,725,433 the form chosen-to be claimedspecifically is that in which the roadanoe X3 is an ui'itu'ried reactance hridg irigo'ornmon terminals of the two couples.

1 present application is directedparticularly to that embodiment of the invention i1-1-us -"too specific'ally in Figure lei-,- in which the co rfiori reactance- X3 is a mutual inductance serving to couple the two circuits including LICI- and inductively, which circuits, so coupled, are aptly described as cascaded tuned or tunable circuits; preferably adjusted to" individual resonance at the carrier frequency of the modulated sighall wave; The circuits are described as cascaded in the sense that energy is transferred from one circuitto the other by their coupling throughthe mutual reactance. The couplingreactiohs between said circuits LlC1X3 and L2C2X3, oporae irig through the reactance of the mutual induct-' ance X3, are utilized to effect the transfer or modulated radio frequenc energy with u iiiformity' and selectivity, and preferably with substantial uniformity through a fre uency range determined by the coefiicient of coupliffg; or the magnitude of mutual reactance with re spect to the other reactan'ces of the systen'i, and preferably also through a range of frequencies substantially coextensive with the limits of some frequency modulation of said energy, as" hereinafter explained.

In using my band selector unit as a fr'eqiie'iiey selector the impressed electro-niotive force may be appiiedin any suitable way,- shown SCE-' matioauy by the e'lectro-motive element E iii-the diagrams, Figures 1 and 1a and the output or the unit may be taken off in any suitable wanes; for example, by means ofa pick-up coil s coupled to the inductance L2 as shown. other specific rheans of applying and takingoff the sieha-l energy are shown in Figures-4, 5 and 6. r

' The operation of the band selector unit may to more" readily understoodby reference" to the vector diagram Figure 2; Let the currents set up By the impressed electro motive fo'r'c'efil thethree branches X1, X2' and :Q be I1, 1'2 and: re

resne ti y- 1 4353. hree I Q S. are QO l id: red-posi ive w en. h ow n e rec i n from the common point a of the branches; to the. o mon point b-q c he t rren flowing into orout of points a and b must be zero ,the current 13in the common reactance X3 must be equal and opposite tothe vector sum of currents I1 and I in the other two. branches, This relation is shown by the vector diagram Figure 2, c being the phase angle between the cnrrents I1 and I2.

4 This phase angle varies from zero to 180degrees in the following manner, depending; upon the; frequency of the impressed electro-motive fo ce-E- Eor any givenband selector there is a critical frequency F1, at which the inductance and capacity reactances L1, C1 and L2 02 of the branchesX and X; are balanced in themselves.

Thepverall reactance of the circuit C1, L1, L2 and C will'then be zero, thecurrent will be in phase with the electro-motive force and its magnitude will depend upon the effective resistance of the system; Thecurrents I1 and I2 will then be in substantially opposite phase relation, considered from the junction points a and b, the angle 1 will be approximately 180 degrees, and the current I3 will be approximately zero, the resistance of the system being considered small.

There is another critical frequency, F2, at whichthe unbalanced reactance of the branches X1, X2 in parallel is equal and opposite to the reactance of the branch X3. The reactancesof the;system as a whole are thus balanced if the cnrrentsh and I2 are in phase, the phase angle 1 being zero, in which case I3 will be approximately, equal to the arithmetical sum of I1 and 1 the effect of resistance being considered small. At any frequency between these limits F1 and Fathe unbalanced reactance of the branches X1 and X2 will have a value intermediate between zero and, X1, the phase angle c will lie between the limits 180 degrees and zero, and the current I3 will; adjust itself between the limits zero and 2I1. the value of X3 is sufliciently small in relation to theotherreactances, the current I2 will be substantially constant at all frequencies between these limits.

At frequencies above or below these limits, the combined reactance of, the branches X1 and X2 will be greater than X3 or of opposite sign to X3, as the case may be, so that X; cannot balance the, unbalanced reactances of X1 and X2 and; theover-all reactance of the system as a whole is large, and this unbalanced reactance will reduce the current in I2 to a small value. The band selector thus is responsive to and transmits with substantial equality all frequencies included in the band lying between the limiting frequencies F1 and F2, and effectively excludes all frequencies outside this band.

It theresistance and other losses of the system are low, as they are preferably, the cut-off at the limiting frequencies is very sharp, and the frequency characteristic of the band selector unit hasthe. form shown in Figure 3.

The width of the band depends upon the relation of the reactance X3 to the other reactances o f-the system. Thus, if X3 is an inductance, as shown in Figures 4-, 5 and 6, the bandwidth dependsupon the relation of this inductance to the inductances -L1 and 112. If the reactance X3 is a capacity, the band width is determined by the relation of the capacity reactance ofXa to. the

If the, resistance of the system is low and capac y reactance. Q l r 2e In h s whe the common reactance is; a mutualinductance'; as shown in Figure la, therelation is similarto that existing in the case of a simple inductance.

Ingeneral the widthof the band expressed as a fraction of' the mean or carrier frequency, is equalto the ratio of thereactance X; to the balanced reactances of the branches X1 and X2 very approximately. Thus whenX1 and X2 are equal andiQ is aninductance having the value L3, the band width isiequal to L3/L1. When X; is a capacity having the valueCa, the band width is (ii/(33, When1X3 isamutual inductance having the;valu e M3, the bandwidth is M3/L1. To cite a specificexample in the case of broadcast reception at a carrier frequency of 1,000 kilycycles with a band widthof 20 kilocycles, the limiting frequencies are 1,010 and 990 kilocycles and the ratio of 1a or M3 to L1 (or C1 to C3, as the case may be) becomes 2 to 100. That is, in the specific embodiment of the present application M3 is equal, to 2% of; L1. Itwill be understood that this example ismerely illustrative, and that the quantities employed may be varied over wide limits to suit the particular case in hand.

The. band width may. be determined within reasonable limits by, choice of the. relation of the common, reactance X; to the other reactances.-. If Xgis made too large the band loses some of its, uniformity, andshowsa depression or valley at the middle. Inpractice, however, the band is substantially uniform when designed for the f requency range represented by a modulated radio si nal wave for example if the system is designed to transmit a band 20 kilocycles wide, which includessubstantially all the side band frequenciesof a modulatedwave. By makingX; variable the band width may be adjusted, at will, to be broad or narrow as conditions or the convenience or pleasure of the operator may, require.

It is of interest to note the relation of the band characteristic, of the band selector unit to the characteristic of a tuned selective circuit. Thus ifthe mutual reactance X3 is omitted and the, two branches X1 andX2 are connected to.- gether, they constitute a resonant circuit tuned toa certain frequency F1, this being one of the limiting frequencies of the band of the selector; unit. The resonance characteristic curve of such a tuned circuit is shown by the dotted linesv in Figure 3 in its characteristic sharplyv peaked form.

When the common reactance )Qis added to: the system the curve takes the bandform shown in full lines, the limiting frequency F1 corresponding to the natural. frequency, of the tuned, circuit; and the limiting frequency Fzbeing, beloworabove this frequency, depending upon whether the reactance X3 is inductive or capacitive,

When the reactance X3 has a suitable small value in reference to the other reactances, the widths of'the two curves at thebase are substantially the same, showing that. the uniform band reception is achieved without any loss of selectivity, butrather with a noteworthy gain.

It will be noted that the gradient of the cut-off in the band characteristic is much sharper than the slope of the resonance curve, since at any frequency outside the band X3 becomes an element of small reactance across the then large The curves shown in Figure 3' are reproduced from records made by an oscillograph of the performance of an actual-apparatus at a-fre quency of 600 kilocycles. a 7

' The band of reception may be readily adjusted in the frequency scale byvarying the capacities CiC2 or the inductances 111112 or both. Usually X3 may remain constant. For example the capacities C102 may-be variable condensers of the usual type, preferably equal, and operated by a single or common control. The band frequency of the system may thus be adjusted to any point in the frequency scale within the limits determined by the ratio of the maximum and minimum capacities of the condensers. In this case if the reactance X3 is an inductance of constant value, the band width, considered as a fraction of the mean frequency, is constant, being determined by the ratio of the constant inductances.

Similarly if the frequency is adjusted by varying the inductances, as it may readily be, for example, by inserting similar short circuiting rings or tubes in the inductance coils, the frequency of the band may be adjusted at any point within the limits determined by the greatest and least value of these inductances. In such case if the reactance X3 is a capacity, the band width, expressed as 'a fraction, will be constant, whatever the position of the band in the frequency scale.

While the inductances and capacities may both be made variable it is usually preferable to make one pair of reactances, for example the inductances, constant and similar. The other pair of reactances which are of opposite sign, e. g. capacitive, in the case assumed, are also preferably made similar and similarly variable. It is usually desirable to make the band forming reactance X: of the same sign as the fixed reactances, thus if the fixed reactances are inductive, X3 will be an inductance; if the fixed reactances are capacitances, X3 will be a capacitance. Inthe case, if Xiis constant, the band width, expressed as a fraction, would be constant as above explained. In general the reactive coupling, due to the reactance X3, between the component resonant circuits or reactive couples should be of sufiicient magnitude to make the frequency response of the system broader than that of the individual circuits, as illustrated in Figure 3, though the variation of the coupling may be used to narrow V the response to any desired degree permitted by the other constants of the system.

By making X3 variable as heretofore noted any desired relation of band width to the frequency may be secured.

In Figure-4 I have shown one of my band selector units employed as a frequency selector in a radio receiving system. The reactive couples X1 and X2 and the common reactance X3 are indicated by the same symbols as in the generalized schematic diagram Figure 1. The band selector unit is associated with the antenna or collector A by a primary coil P coupled with the inductance L1 of the reactive couple X1. 'The band selector unit may be associated with an aperiodic amplifying and detecting system, such as the detector D and audio frequency amplifier A1 in any suitable way. I prefer to form this association by an adjustable aperiodic coupling which will give control of the strength of signal impulses applied to the system. A convenient arrangement for this purpose is an aperiodic pickup coil S which is in Variable inductive relation with the inductance L2 of the band selector. Since the purpose of this coil is to derive from the currentin L2 an electro-motive force which is applied to the detector, tuning or frequency adjustment is not necessary. It is sufiicient to have the magnetic circuits of the two coils interlinked. By varying the degree of interlinkage, the electro-motive force applied to the detector may be varied from zero to a maximum. The maximum occurs when the coils are closely coupled, and the minimum when their fields are not interlinked at all.

The antenna coil P is preferably closely coupled to the inductance L1. Usually I prefer a step-up ratio of turns, i. e. the number of turns of the antenna coil P is less than the number of turns of the inductance L1. In the case of such a step-upratio the effective capacity introduced into the reactive element X1 by the antenna is less than the antenna capacity, in proportion to the ratio of turns. For this reason, and for other reasons that will be understood, this inductive coupling is usually preferable to connecting the antenna and ground directly across the capacity C1.

- The-effective capacitance introduced by the antenna, or in general the effective reactance introduced by the collector, into the reactive element X1 is an indeterminate factor which if not compensated, would unbalance the symmetry of the system, and, if large enough, would distort the band characteristic. A feature of the present invention which avoids such unbalance and distortion is the introductionof a compensating reactance in one of the reactive couples corresponding to the indeterminate reactance introduced into the other reactive couple. For example, in the case where the element that introduces the indeterminate reactance is a collector and the reactance introduced by the collector is capacitive, as shown in Figure 4, symmetry may be restored by introducing a compensating capacity Cc, which is shown in parallel with the capacity 02. This capacity may be adjusted to compensate for any desired value of the capacity of the collector, but I prefer to make it a fixed capacity larger than the largest value of the effective capacity that will be introduced into the element X1 by the collector. I then employ an adjusting capacity Cx in parallel with the capacity C1, to make up the difference between the compensating capacity Cc and the effective capacity introduced into the system by the collector.

It will be readily understood that any equivalent device for producing similarity in two circuit'elements will be applicable to the specific case of the two circuit elements, one of which includes a collector, in the band selector unit of the present invention,

I In the arrangement shown in Figure 4, theposition of the band'of reception in the frequency scale is determined by adjusting the capacities or condensers C1 C2 simultaneously by'a common control movement, whereby the frequency of the band of reception may be changed at will without altering its uniform band character.

In Figure 5 I' show one of my band selector units employed as a preliminary selector, or preselector, with a coupled collector, in conjunction with a band amplifier of the type set forth in Patents Nos. 1,666,518, April 17, 1928, 1,682,874, September 4, 1928 and'1,730,987, October 8-, 1929. This is a very desirable improvement over the combination including a tuner, set forth in my Patent No. 1,730,987.

capacities of the band selector unit and the band amplifier, the. frequency characteristics may: be made to coincide sothat they may be adjusted in the frequency scale byv a single or common control means, asshown and fully explained, in my former application.

A very important characteristic of the described band selector system is that any number of band selector units may be employed in cascade, thus greatly increasing the selectivity of the system, without narrowing the response curve. When the conventional system of tuning by resonance is employed the use of synchronously tuned circuits in cascade inevitably sharpens the response curve, thus trimming the side bands and destroying the fidelity. W'hen band selector units are used in cascade any desired degree of selectivity may be obtained without narrowing the effective band of response. There is no diminution of signal strength at any part of the useful reception band, but the use of successive units serves to steepen greatly the gradient of the cut-off, thus improving selectivity.

The use of a selector unit of the type described as a pre-selector in advance of the first amplifier tube, is also important. When a single tuned circuit is used, as is customary at the present day, a powerful signal of foreign frequency introduces forced oscillations which are impressed on the grid of the first amplifier tube and modulate the desired signal oscillations, producing cross modulation or cross talk, so that when the desired signal is tuned in the interfering signal is heard superimposed upon it.

Such cross modulation is prevented by the use of a pro-selector unit of the type described, which is double tuned by means of the two reactive couples X1 X2 and reduces the grid swing of the first tube due to forced oscillations to a point that is not sufficient to modulate perceptibly the desired signal wave.

Both of these features are of great practical value and both are embodied in the arrangement shown in Figure 6 which includes a plurality of double tuned selector units, one being employed as a pre-selector coupling the collector, here shown as a loop collector,,with the amplifier, the others being employed as interstage coupling elements of a radio frequency amplifier.

This arrangement includes a plurality of amplifier tubes A1, A2, D, which are coupled in cascade, but the interstage coupling means in this case-is not a single tuned transformer unit as in Figure 5, but a double tuned selector unit. The coupling means in each case comprises two reactive couples X1 and. X2, each of which includes an inductance and a capacitance. The inductances or the capacitances or both are variable for the purpose of frequency selection. In the preferred arrangement shown the inductances are fixed and the capacitances C1 and C2 are variable, These two reactive couples are associated with each other by means which permits the transfer of oscillatory energy between them, which means, in the arrangement shown, comprises the reactive element X3 whose reactance is common to both couples. Preferably the reactance X3 is so related to the other reactances in the system as to balance the reactances of the complete selector unit at a plurality. of frequencies so that the system is responsive to all the frequencies included in the transmission band, as above explained.

The output of the first tube, such as A1, is im-- pressed on a selector unit X1, X2, X3, which serves asv the coupling means connecting the tubes incascade and the. selector unit. is. also operatively connected to the second tube such as A2. The means. shown for impressing the output of the first tube on the selector .unit is the connection c-between the anode of the first tubefand the reactive couple X1 and the connection d serves to connect the second couple X2 to the grid or input circuit of the second tube, impressing signal oscillations thereon,

In the arrangement of this figure three of these selector units are shown, operating in cascade and coupled by the amplifier tubes A1 and. A2, the first being employed as a pre-selector in advance of the first tube A1, and being associated with collecting means, here shown as a loop collector L1, the second being employed as an interstage coupling unit, coupling the tubes A1 and A2 in cascade, and the third coupling the amplifier tube A2 to the detector D.

It will be noted that each of the selector units is double tuned by means of the variable condensers C1 and 02.

It has been stated above that the means shown for impressing the output of the first tube on the selector unit is the connection 0 and that the connection it serves to connect the second couple to the input circuit of the second tube, but it will be obvious that instead of the direct electrical con nection shown, a transformer or other suitable coupling may be employed, as is well known in the art. It is characteristic of the selector unit described that the impedance between the points 0 and b and the points d and b is high at all frequencies within the band of effective response of the system and relatively low at frequencies outside of this band.

The two or more band selector units are preferably made alike, for convenience in mechanical construction. The inductance L of the loop is made approximately equal to that of the inductor L2, and a small compensating inductor L2 added, so that all the units are made symmetrical and all may be similarly adjusted by a single control means.

It will be understood however that complete symmetry is not essential, provided there is such similarity as will give the various band selector units similar frequency characteristics, the variable reactances being similarly variable so that they may be all operated by single control means as shown, which serves to adjust the pre-selector and the interstage coupling unit simultaneously by a single operation, 50 that the frequency response of the several selectors is simultaneously and similarly adjusted and the system as a whole is made responsive to any desired signal frequency or band of frequencies.

It will be understood that other modifications and applications of the system may be made without departing from the essential principles of the invention.

I claim as my invention:

1. The method of selectively receiving the transmission band of a modulated signal wave, which consists in applying the signal energy to a receiving system, balancing the reactances of the system in pairs atone frequency, balancing the unbalanced reactances of the pairs at other limiting frequencies by a mutual inductance operating between the pairs, thereby receiving with substantial uniformity all frequencies in the band included between these limiting frequencies, and varying the width of the band of reception by varying the magnitude of the mutual inductance.

2. In a system for receiving the transmission band of a modulated wave, a band selector unit comprising two reactive couples, each having reactances that are partly balanced at the frequencies included in the transmission band, a mutual inductance operating between the couples whose value is so related to the reactances of the couples that the unbalanced portion of these reactances is balanced and. the system is made responsive to all frequencies within a definite band, and means for varying the width of the band of response by varying the magnitude of the mutual inductance.

FREDERICK K. VREELAND. 

